Nitrogen Fertilizer Recovery by Grain Sorghum in Monoculture and Rotation Systems

Abstract

AbstractGrain sorghum [Sorghum bicolor (L.) Moench] has become a major dryland crop for the Great Plains, but information on production in rotations is limited. This study was conducted to determine N fertilizer recovery and use by grain sorghum in monoculture and rotational systems. Grain sorghum was grown under rainfed conditions on a Sharpsburg silty clay loam (fine, montmorillonitic, mesic, Typic Argiudoll) in: (i) continuous grain sorghum monoculture, (ii) a 2‐yr soybean [Glycine max (L.) Merr.]‐grain sorghum rotation, (iii) a 4‐yr rotation of soybean‐corn [Zea mays L.]‐oat [Avena sativa (L.)] + clover [80% Melilotus officinalis (L.) and 20% Trifolium pratense (L.)]‐grain sorghum, and (iv) a 4‐yr rotation of oat+clover‐corn‐soybean‐grain sorghum at Mead, NE. Broadcast applications of 15N‐depleted NH4NO3 were made at 90 and 180 kg N ha−1 in 1985 and 1986 to evaluate N fertilizer recovery. Fertilizer N recovery determined by isotopic methods was significantly higher for grain sorghum in monoculture (64.9%) vs. grain sorghum in rotation (54.9%). Fertilizer N recovery estimated by the difference method ranged from 118.5% in continuous grain sorghum to 9.1% in sorghum following oat+clover. Differences in N‐recovery results by the isotope and difference methods indicated N fertilizer applied to grain sorghum in various cropping systems appeared to be entering different organic soil‐N pools. These results substantiate the importance of the mineralization‐immobilization turnover (MIT) effect in N‐isotope experiments and the necessity for careful interpretation of N‐recovery results, especially between complex cropping systems.